The manufacture of many products requires precise control over temperature and temperature changes. For example, the manufacture of microelectronic devices, such as integrated circuits, flat panel displays, thin film heads, and the like, involves applying a layer of some material, such as a photoresist, onto the surface of a substrate (such as a semiconductor wafer in the case of integrated circuits). Photoresists, in particular, must be baked and then chilled to set or harden selected portions of the photoresist during processing. The baking and chilling steps must be precisely controlled within exacting temperature constraints to ensure that the selected portions of the photoresist properly set with good resolution. Other products and processes involving exacting temperature constraints include medical products and processes including drug preparation, instrument sterilization, and bioengineering; accelerated life testing methodologies; injection molding operations; piezoelectric devices; photographic film processing; material deposition processes such as sputtering and plating processes; micromachine manufacture; ink jet printing; fuel injection; and the like.
Baking and chilling operations for microelectronic devices typically involve cycling a workpiece through a desired temperature profile in which the workpiece is maintained at an elevated equilibrium temperature, chilled to a relatively cool equilibrium temperature, and/or subjected to temperature ramps of varying rates (in terms of .degree. C./s) between equilibrium temperatures. To accomplish baking and chilling, previously known bake/chill operations have included separate bake and chill plates that have required the use of a workpiece transport mechanism in order to physically lift and transfer the workpiece itself from one plate to the other. This approach presents a number of drawbacks. First, workpiece temperature is not controlled during transfer between bake and chill plates. Second, the overall time required to complete the bake/chill process cannot be precisely controlled, because of the variable time required to move the workpiece to and from the respective plates. Third, the required movement takes time and thus reduces the throughput of the manufacturing process. Fourth, the cost of equipment is higher than necessary because the apparatus requires extra components to handle the workpiece during transport from plate to plate. Fifth, the mechanical move from plate to plate introduces the possibility of contaminating of the workpiece. Thus, it would be desirable to be able to accomplish both baking and chilling without having to physically lift and transport the workpiece itself from a bakeplate over to a separate chill plate and vice versa.
Baking typically involves heating a workpiece up to a specific elevated, equilibrium temperature and then maintaining the workpiece at that particular equilibrium temperature for a defined period of time. Throughput of the manufacturing process is affected by the rate at which the workpiece can be heated up to the equilibrium temperature. Slower temperature ramp rates require longer times to reach the equilibrium temperature and therefore result in lower manufacturing throughput. Similarly, chilling typical involves cooling the workpiece from a relatively high temperature down to a relatively low, chill equilibrium temperature. Again, slower chilling rates require longer times to complete chilling and therefore also cause lower manufacturing throughput. Accordingly, in order to improve manufacturing throughput, it would be desirable to increase the rate at which the workpiece temperature can be changed during baking and/or chilling so that the workpiece can be brought to the heating or chill equilibrium temperatures faster. It would also be desirable to be able to accurately control the rate at which workpiece temperature is changed during baking and chilling.
Even if the bake and chilling rates were to be increased, it would still be necessary to accurately control the temperature of the workpiece throughout the bake/chill process to make sure that the exacting temperature specifications for workpiece production are satisfied. For example, if chilling and/or baking rates as fast as 1.degree. C./s to 50.degree. C./s, preferably 5.degree. C./s to 15.degree. C./s, were to be used, the control approach would need to be agile enough to be able to control the workpiece temperature commensurately with such rapid temperature changes. The conventional control approaches currently used for heater control generally lack the requisite agility to be able to keep up with a bake/chill station having such capabilities.
With respect to the manufacture of microelectronic devices, current practices also involves using a relatively massive bakeplate. Massive bakeplates typically require relatively long periods of time, e.g., periods up to 30 minutes, in order to change from one temperature and come to equilibrium at a new temperature. Accordingly, to avoid having to wait so long each time a baking temperature is changed, current practice often involves using multiple bakeplates set at different equilibrium temperatures, a workpiece handler to transfer workpieces from one bakeplate to the next, and fixed, slow, temperature ramp rates.